On Thu, 28 Jul 2016 00:18:14 +1000
Michael Ellerman <m...@ellerman.id.au> wrote:
> From: Kevin Hao <haoke...@gmail.com>
>
> We do binary patching of asm code using CPU features, which is a
> one-time operation, done during early boot. However checks of CPU
> features in C code are currently done at run time, even though the set
> of CPU features can never change after boot.
>
> We can optimise this by using jump labels to implement
> cpu_has_feature(), meaning checks in C code are binary patched into a
> single nop or branch.
>
> For a C sequence along the lines of:
>
> if (cpu_has_feature(FOO))
> return 2;
>
> The generated code before is roughly:
>
> ld r9,-27640(r2)
> ld r9,0(r9)
> lwz r9,32(r9)
> cmpwi cr7,r9,0
> bge cr7, 1f
> li r3,2
> blr
> 1: ...
>
> After (true):
> nop
> li r3,2
> blr
>
> After (false):
> b 1f
> li r3,2
> blr
> 1: ...
>
> Signed-off-by: Kevin Hao <haoke...@gmail.com>
> Signed-off-by: Aneesh Kumar K.V <aneesh.ku...@linux.vnet.ibm.com>
> Signed-off-by: Michael Ellerman <m...@ellerman.id.au>
> ---
> arch/powerpc/include/asm/cpu_has_feature.h | 22
> ++++++++++++++++++++++ arch/powerpc/include/asm/cputable.h |
> 6 ++++++ arch/powerpc/kernel/cputable.c | 20
> ++++++++++++++++++++ arch/powerpc/lib/feature-fixups.c | 1 +
> 4 files changed, 49 insertions(+)
>
> v3: Rename MAX_CPU_FEATURES as we already have a #define with that
> name. Define NUM_CPU_FTR_KEYS as a constant.
> Rename the array to cpu_feature_keys.
> Use the kconfig we added to guard it.
> Rewrite the change log.
>
> diff --git a/arch/powerpc/include/asm/cpu_has_feature.h
> b/arch/powerpc/include/asm/cpu_has_feature.h index
> ad296b2f1d84..18e60e61bea9 100644 ---
> a/arch/powerpc/include/asm/cpu_has_feature.h +++
> b/arch/powerpc/include/asm/cpu_has_feature.h @@ -11,10 +11,32 @@
> static inline bool __cpu_has_feature(unsigned long feature)
> (CPU_FTRS_POSSIBLE & cur_cpu_spec->cpu_features & feature)); }
>
> +#ifdef CONFIG_JUMP_LABEL_FEATURE_CHECKS
> +#include <linux/jump_label.h>
> +
> +#define NUM_CPU_FTR_KEYS 64
> +
> +extern struct static_key_true cpu_feature_keys[NUM_CPU_FTR_KEYS];
> +
> +static __always_inline bool cpu_has_feature(unsigned long feature)
> +{
> + int i;
> +
> + if (CPU_FTRS_ALWAYS & feature)
> + return true;
> +
> + if (!(CPU_FTRS_POSSIBLE & feature))
> + return false;
> +
> + i = __builtin_ctzl(feature);
> + return static_branch_likely(&cpu_feature_keys[i]);
Just a reminder to add a BUILD_BUG_ON(!__builtin_constant_p(feature));
for this.
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